VB script function not firing from asp:button click

Hi,
I have a vbscript function on the page to open the calendar.I am trying to fire this from button click event.
I am getting this error 'getdate' is not a member of 'ASP.default_aspx'.
Function getdate
frm1.txtdate.value = window.showModalDialog("calendar.aspx","","dialogHeight: 200px;dialogWidth: 400px;status:no;" )

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

Hi robert_Schutt
I tried the OnClienClick="getdate();" but it is not working.The calenadar is not showing up.there is no error also.

The getdate function defined in the head section of aspx page.
To maintain consistency I have to use the asp button controls.I can't use HTML button controls Because few more asp buttons are there on the same row.